# std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::value_compare

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< cpp‎ | container‎ | flat map

class value_compare; |  | (since C++23)

`std::flat_map::value_compare` is a function object that compares objects of type `std::flat_map::const_reference` (a pair of references to the stored key/value) by comparing the first components (i.e. keys) of the passed pairs.

### Member objects

Member name | Definition
`comp` (private) | the comparison function object of type `std::flat_map::key_compare` (exposition-only member object*)

### Member functions

(constructor) (private) | constructs `value_compare` object (public member function)
operator() | compares two values of type `value_type` (public member function)

## std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::value_compare::value_compare

private: value_compare( key_compare c ); |  | (exposition only*)

Initializes the underlying instance of the comparator `comp` to c.

### Parameters

c | - | a comparator to assign

## std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::value_compare::operator()

bool operator()( const const_reference& lhs, const const_reference& rhs ) const; |  |

Compares lhs.first and rhs.first by calling the stored comparator `comp`.

### Parameters

lhs, rhs | - | values to compare

### Return value

comp(lhs.first, rhs.first)

### Exceptions

May throw implementation-defined exceptions.

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